1. Field of the Invention
The present invention relates to a method for creating ultra-fine particles of a material using high-pressure fluid and a mill for implementing the method. More particularly, the present invention relates to a method and- apparatus for subjecting particles to a high-pressure liquid jet, high turbulence condition, cavitation and collision to comminute the particles.
2. Related Art
Comminution may be defined as either a single or multistage process by which material particles are reduced from random sizes by crushing and grinding to the size required for the intended purpose.
Size reduction in comminution machines relies on three different fragmentation mechanisms: cleavage, shatter, and abrasion. It is commonly stated that only three percent of the energy used in fragmenting solid particles goes into the creation of new surfaces. Thus, current comminution technology is both energy-intensive and inefficient.
During milling of material, to create a fracture in the particles of material, a stress must be induced which exceeds the fracture strength of the material. The mode of fracture and the path that it follows depends on the material, the shape and structure of the particle, and on the way and rate at which the load is applied. The way in which the load is applied will control the stresses that induce fracture extension or growth within the particle. The force used to induce this growth can be one of simple compression, which causes the particle to fracture in tension, whether at a slow or fast rate. Alternatively, the applied load may be in shear, such as is exerted when two particles rub against each other, or the load may be applied as a direct tensile force on the particle.
For optimum comminution of hard materials such as minerals a shattering fracture is most beneficial. This occurs when the energy applied to the particle is well in excess of that required for fracture. Under these conditions, very rapid crack growth is induced and will cause crack bifurcation. Thus, the multiplicity of areas in the particle that are simultaneously overstressed will combine to generate a comparatively large number of particles with a wide spectrum of sizes. Shattering usually occurs under conditions of rapid loading (e.g., a high velocity impact) with maximum size reduction occurring around the impact points. According to existing theory, the finest product sizes are generated in the zone around the impact point, when insufficient energy is applied to cause total fracture of the particle. The localized nature of the applied stress and the high energy required for this ultra-fine grinding make this process relatively inefficient.
Conventional milling machines use mechanical crushing or crushing and attriting to break mineral particles into smaller particles. The low efficiency of existing reduction processes is frequently due to the application of stress where there are no particles. The result is that much of the energy input is wasted in non-productive contact between, for example, crushing mechanisms or between a crushing mechanism and the mill wall, both of which lower the overall energy efficiency of the process.
Further, for brittle materials, there is a considerable difference between the values of uniaxial compressive strength and tensile strength of the material. Thus, the amount of energy which must be consumed in breaking the mineral into small particles under compressive loading is substantially higher than that required if the material can be induced to fail under a tensile stress. To induce simple tensile failure, high pressure liquid jets or different liquid jets have been used in comminution processes.
Size reduction involves rupturing the chemical bonds within the material in order to generate new surfaces. Thus, the chemical processes associated with fracture will significantly affect the energy required to induce this fracture. This influence extends beyond the bonds themselves to include the surrounding environment. For example, the presence of liquid at the crack tip will lower the forces required to expand the crack and improve efficiency, especially where the liquid contains inorganic ions and organic surfactant. One explanation for this effect is that the additives penetrate into microcracks ahead of the major crack front and thus take part in the highly reactive events that occur during fracture. Because the capillary flow of these liquids into the material ahead of the main front runs at the velocity of crack propagation it provides a means of transmitting energy more easily within the crack tip zone. A high-pressure liquid jet containing chemical additives creates extremely dynamic conditions in which microcracks grow ahead of the main failure plane and become pressurized, thereby enhancing any chemical changes which might occur.
For use in liquid-fueled power plants, it is necessary to produce a homogeneous, pumpable suspension of coal that will not settle in delivery lines and which burns at the required rate. Therefore, the coal must be ground from the "standard plant size" to a diameter below 40 microns. Among the many milling methods used for this process the finest product is achieved by the use of autogenous attriting machines. The distinguishing feature of these machines is that size reduction is effected by particles impacting upon each other, after being given the necessary energy to induce fragmentation through a solid or liquid impeller. Included in this class are the following systems: (1) Buhrstones--which cause comminution through an abrasion action; (2) Colloid Mills--in which comminution occurs by collision between particles; (3) Fluid Energy Mills--in which particles interact upon one another; and (4) Sand Grinder--in which particles are reduced by contact with sand particles.
The advantage of the conventional equipment is that the product is reduced to very small sizes (below 40 microns) and distributed within a narrow size range. The equipment, however, can only operate, at any one time, with small quantities of feed, and the initial feed size of particles lies in the range between 0.5 inches and 50 microns, depending on the type of unit. For the sand grinder, for example, the feed stock should already be crushed to below 70 microns. A much greater disadvantage for this type of machine is the very high power consumption required to achieve the required crushing.
The energy required to achieve a given size reduction increases as the product size decreases. This increase is due to many factors and is a consequence not only of the type of mill or the microscopic condition of the material, but also relates to the mechanism of failure at the individual particle level. This is obvious because fragmentation in a chamber is partly brought about by an interaction between the particles and the chamber wall.
In such situations, the treatment of individual particles requires special attention. For example, a coal particle is anisotropic, heterogeneous, and extensively pre-cracked. Physical properties of coal vary as a function of the degree of metamorphism of the coal particle. Because of the organic nature of the material, this means that different properties may be encountered, even within a single particle. Under such a situation an analytical approach to coal fragmentation is very complex.
The efficiency of coal comminution depends on the ability to take advantage of the anisotropy of coal particles which is, in turn, a function of the internal structure. However, with liquid jet comminution, failure occurs on the basis of differential coal porosity and permeability, as these properties control the specific rates of liquid absorption, which directly influence the rate of disintegration.
Experiments conducted with shaped explosive charges to investigate fracture formation in coal showed that there is intense fracturing of coal near the jet path, with this zone of fracture usually bounded by joints, bedding planes and cleat planes. The coal breaks into large and small pieces, usually parallel following natural cleavage planes. Beyond this intensely crushed zone, some large fractures were observed. These crossed joints and traveled long distances, while fractures originating at the base of the jet penetration also crossed bedding planes and extended the zone of influence deeper into the target material.
Comminution technology can also be used to comminute organic materials. One example of such a material is wood. These organic materials are generally softer than the inorganic materials discussed above. In the case of organic materials, the impact of the waterjet causes a shearing force to occur to break apart the material, rather than the crack propagation discussed above.
Conventional comminution technology is both energy intensive and inefficient. Up to 97% of the energy consumed during the operation of conventional size reduction devices can go into non-productive work, with only 3% of the energy input then being used to create new surfaces. Comminution is thus an appropriate target for significant energy savings, since the tonnages of materials involved in the size reduction operations are so great that even small improvements in comminution efficiency would provide considerable savings in energy and mineral resources.
Further, conventional comminution devices are very expensive and wearing process of the friction parts are very significant and costly.
Through study it has been found that a high-pressure liquid jet has an excellent, and in some ways a unique, ability to improve material disruption. Such a capability is due to the following features:
A liquid jet of 10,000 psi pressure moves at approximately 1,332 ft/sec, with a narrowjet diameter providing a concentrated energy flux input to the target. PA1 The high energy density of the liquid jet is concentrated in a very small impact zone, while the intense differential pressure across the jet enhances microcrack generation and growth. PA1 Subsequent to the initial impact, the jet stagnation pressure forces liquid into the cracks and microcracks. It develops a hydromechanical jet action in these cracks and creates an increasingly dense network of cracks in the walls of the cavity created. PA1 Rapid jet penetration into pre-cracked minerals can be enhanced by the use of surface active agents, which will also work to further comminute the coal and retreat any mineral matter in the coal. PA1 In those circumstances where a coal/oil mixture (COM) is required, the liquid jet can be changed to an oil jet, for example, to eliminate the intermediate drying process. PA1 The separation of mineral matter from coal is improved by use of pressurized liquid jets. On occasion, this separation is enhanced by the differential response of the constituent materials to the jet attack which can facilitate separation of the resulting particles on the basis of the size differential in the grain or crystal sizes of these materials. PA1 There is a reduced expectation of mechanical wear or process contamination of the product.
Conventional jet energy mills have a size reduction factor of approximately 50. This means that conventional mills can reduce the size of a particle so that the product size of the final, resultant particles is 50 times smaller than the original feed size of the particles. What is needed is a mill that makes efficient use of high-pressure liquid jets in the comminution of materials into ultra-fine particles.